Agriculture Reference
In-Depth Information
stem plate and sprout leaves, commencing before sprouting and continuing to
increase as sprouting occurred. These fractions decreased in the inner scale
leaves, suggesting the mobilization of storage carbohydrates from their 'source' in
the scale leaves via the stem plate to a 'sink' in the sprout leaves, where they were
utilized in the biosynthesis and respiration of sprout leaf growth (Yasin and
Bufler, 2007).
Effects of harvesting time
Several studies have shown that, in order to maximize the time to sprouting in
store, it is best to harvest before the bulbs have reached their maximum weight,
probably when 50-80% of plants have reached the stage of 'soft-necks' and
foliar collapse (Komochi, 1990). Harvesting later frequently results in poor
skin quality and, as described above, the loss or puncturing of skins in stored
bulbs accelerates sprouting.
The role of growth regulators
Growth-inhibitory substances have been extracted from dormant bulbs by a
number of workers (Komochi, 1990). Stow (1976) found growth inhibitors in
the leaves of onions during bulbing, and postulated that these were
translocated to the bulb and were responsible for maintaining its subsequent
dormancy. He found that early defoliation or leaf desiccation of bulbs resulted
in earlier sprouting, and suggested that this was because the inhibitory
substance had not been fully translocated from leaf to bulb before defoliation
(see Fig. 7.11).
Abscisic acid (ABA) was detected within bulbs, but the amount corre-
sponded to only 10-20% of the growth-inhibitory activity found in bulb extracts.
Increases in ABA concentration and its subsequent decline correlate well with
the onset and decline in dormancy in bulbs of
A. wakegi
(see Fig. 4.46). Chope
(2006) found that ABA concentrations in onion bulbs declined exponentially
during storage and that sprouting occurred at the minimal ABA concentration,
but bulb soaks or pre-harvest sprays with ABA or an ABA analogue (8
-
methylene ABA methyl ester) failed to increase internal ABA concentrations and
prolong dormancy.
Exposure of bulbs for 24 h to an atmosphere containing 1
l/l of 1-
methylcyclopropene (1-MCP) retarded sprouting in bulbs stored at 4 or 12°C but
not at 20°C storage (Chope
et al.
, 2007). Carbohydrate metabolism was inhibited
in bulbs stored at 12°C. 1-MCP is an inhibitor of ethylene perception by plant
tissue, suggesting an involvement of ethylene in onion bulb dormancy.
Continuous exposure to 10
l/l of exogenous ethylene gas inhibits the elongation
of sprout leaves in stored bulbs (Bufler, in press, b) and is rapidly being adopted to
prevent sprouting in commercial stores (see 'Controlled Atmosphere Storage',
below).
Levels of growth hormones are low in dormant bulbs but, as sprouts start to
develop, increases in cytokinin - followed by gibberellin, then auxin - activity
